President Donald Trump gave his official approval Tuesday for Montenegro to join NATO, marking another step forward in the tiny Balkan country's quest for NATO acceptance.

The White House says Trump looks forward to meeting with Montenegro and other NATO leaders next month in Brussels to welcome the 29th member of the alliance.

The White House statement said Montenegro's accession will signal other countries seeking to join NATO that "the door to membership in the Euro-Atlantic community of nations remains open and that countries in the Western Balkans are free to choose their own future."

The U.S. Senate voted overwhelmingly last month to support Montenegro's NATO bid.

Trump is scheduled to meet Wednesday with NATO Secretary-General Jens Stoltenberg at the White House.

As recently as January, Trump called NATO "obsolete" because it had not defended against terrorist attacks. He also complained other NATO countries are not paying their fair share for defense.

"A lot of these countries are not paying what they are supposed to be paying, which I think is very unfair to the United States," Trump told The Times of London. "With that being said, NATO is very important to me. There are five countries that are paying what they are supposed to. Five. It is not much."

Russia has described Montenegro's NATO membership as a "provocation" due to the country's geographical proximity to Russia. The Kremlin has long seen the Balkans as inside its "sphere of influence."